Annealing treatment and microstructure analysis on the electroplating Ni-B alloys – POSTER

Yu Ing-Song - National Dong Hwa University (Taiwan)

Owing to the high corrosion and wear resistance of the electroplating Ni-B deposit, it has promising potential to replace the hexavalent Cr deposit which is widely applied for functional surface coating but highly toxic. Ni-B deposits were electroplated on the medium-carbon steel bar in the Ni-B plating bath containing 6 g/L trimethy-lamine borane and 5 g/L saccharin. After electroplating process, Ni-B deposits were annealed at 200, 300, 400, and 500 degree C for 30 min, respectively. And then, samples were also standing still at room temperature for about 60 and 300 days. Hardness values of prepared Ni-B deposits were evaluated, and their microstructures were studied with transmission electron microscope (TEM). Experimental results show that the hardness of an annealed Ni-B deposit increased with an increasing annealing temperature up to 400 degree C. The highest hardness of 1071 HV was obtained at 400 degree C. In the condition of annealing temperature from 400 to 500 degree C, the hardness of Ni-B deposit dropped obviously. The hardness value of Ni-B deposit decreased apparently to 615 HV after annealing at 500 degree C. The observation of TEM dark-field images revealed that the hardening and softening mechanism of annealed Ni-B deposits attributed to precipitation and growth of NiB phase. Ni-B deposits had the crystal structure with the grain sizes lower than 20 nm after annealing below 400 degree C. However, abnormal grain growth with few grains larger than few-hundred nanometers was found at the annealing temperature of 500 degree C, which caused the decrease of hardness for the deposits. Self-annealing behavior through abnormal grain growth could be observed from as-plated and annealed Ni-B deposits with standing still at room temperature for about 60 and 300 days, which led to apparently decrease in hardness.